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12

Sinterability and Dielectric Properties of ZnNb2O6 – Glass Ceramic Composites

Manoj Raama Varma, C. P. Reshmi and P. Neenu Lekshmi Materials and Minerals Division,

National Institute for Interdisciplinary Science and Technology [NIIST],Thiruvananthapuram,

India

1. Introduction

In the new era of communication technology there are revolutionary developments in

satellite communication, global positioning systems and mobile communication systems,

which has helped the developments in multilayer technologies like low temperature cofired

ceramics (LTCC). The microwave electronic devices have achieved significant

miniaturisation, light weight and became very cost effective using LTCC. The characteristic

properties required for dielectric materials which are used in multilayers are (a) high

dielectric permittivity( rε ), (b) high quality factor (Q×f) and (c) low temperature coefficient

of resonant frequency (τf). The size of the resonator is inversely related to the rε .

Dielectric materials should posses near zero temperature coefficient of resonant frequency

(τf) for thermally stable electronic devices [1-7]. Generally most of the dielectric ceramic materials are known to posses the above said

properties but will sinter at temperatures above 1000 oC. Zinc niobates, ZnNb2O6 (ZN) is a

low loss dielectric material with columbite structure having excellent dielectric permittivity,

high quality factor and low temperature coefficient of resonant frequency. Sintering

temperature of ZN is comparatively lower (~1200 oC) [8]. Hence it is widely used as

dielectric resonators in microwave communication devices. In multilayer ceramic structures,

the low melting electrodes such as Ag (melting point ~961 oC), Cu (melting point ~1083 oC)

and Au (melting point ~1064 oC) are co-fired with these ceramic materials [9,10]. In the case

of Ag electrodes, processing temperature of the material must be below 950 oC.

There are several approaches to reduce the sintering temperature of the ceramics viz. (i)

usage of ultra-fine particles/powders as synthesized by wet chemical methods as starting

materials (ii) addition of low melting glasses to obtain a low temperature sintering

composite [11-14]. Glass addition is known to be the most popular and least expensive

method and hence ZN is widely used in ceramic technology.

Even though the ZN ceramics prepared by conventional ceramic route [1-6] shows excellent

properties, high sintering temperature preclude its application potential in the LTCC. Usage

of nano sized ZN powders (instead of micron size powders) in multi layer technology can

bring down the sintering temperature to a lower value. Hence the procedure for preparing

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Advances in Ceramics - Synthesis and Characterization, Processing and Specific Applications

278

ZN ceramic powder as both micron size powders and nanosized powders and the

sinterability of pure materials as well as the glass added ceramics are described in this

chapter. The structural characterisation of the materials can be done using XRD and the

microwave dielectric properties (in the frequency range 2-6 GHz) can be studied and

compared to highlight the effect of particle size on sinterability and microwave dielectric

properties of these materials.

2. Materials and methodologies

ZN ceramic powders can be synthesized using two different well established preparation

techniques such as solid state ceramic method and polymer complex techniques [15-17].

2.1 Synthesis of ZnNb2O6 using solid state synthesis technique

Single phase ZN can be prepared using oxides of Zn (ZnO 99.9+%) and pentoxide of

Niobium (Nb2O5 99.9+%) as raw materials. These oxides can be weighed in stoichiometric

proportion and mixed for 24h in a ball mill, using zirconia balls and distilled water as the

milling medium. The slurry can be dried at 80 oC and the dried powder can be calcined at

850 oC/4h, to get the phase pure ZN ceramics.

2.2 Preparation of ZnNb2O6 using polymer complex method

Zinc acetate [Zn(CH3COO)2 99.99%,] and niobium ethoxide [Nb(OC2H5)5, 99.95% metal

basis] can be used as the starting material for preparing ZN using polymer complex method.

The flow chart, in Fig. 1 shows the various steps involved in the synthesis. 3 mol equivalent

of citric acid can be dissolved in 12 mol of ethylene glycol with continuous stirring for 1h to

form a clear solution. 1 mol of zinc acetate can then be added and stirred for several hours at

80oC to dissolve it completely. 2 mol of Nb(OC2H5)5 can be added to this clear solution with

a stirring speed of 500 rpm until it results in the formation of a thick white gel. The gel can

be sonicated for 2h to obtain the uniform distribution. After sonication the polymeric

precursor can be recovered by desalting with acetone. The dried polymeric precursor can be

calcined at 600oC/4h to obtain the ZnNb2O6 nanopowders.

2.3 Preparation of 60ZnO-30B2O3-10SiO2 (ZBS) glass

High purity ZnO, B2O3 and SiO2 (99.9%) can be used as the raw materials for the preparation

of 60ZnO-30B2O3-10SiO2 (ZBS, sintering temperature is <800 oC). The raw materials weighed

accurately in the stoichiometric proportion are mixed well in distilled water medium using

zirconia balls for 24 h in a ball mill. The slurry can be dried and the powder can be melted in

a platinum crucible at 1000oC for 2h, and the melt can be quenched into cold distilled water

and powdered. This glass powder can be used for the preparation of glass ceramic

composites.

2.4 Preparation of ZN-ZBS glass composites

Appropriate amounts of ZN and ZBS glass (1,3,5,10 wt%) can be mixed using an agate

mortar for 2 hours in distilled water medium and the slurry can be dried and powdered.

3wt% PVA solution can then be added to this mixture as a binder. The dried powder can be

uniaxially pressed using a tungsten carbide (WC) die in the form of cylindrical discs of

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279

Fig. 1. Flow chart for the preparation of ZnNb2O6 nano powders

3 Mol

Citric Acid

12 Mol

Ethylene Glycol

1 Mol Zn(CH3COO)2

Heat Treated at 80o c/2h with stirring

Clear Solution Obtained

2 Mol Nb(C2H5O)5

White Gel

Sonicated for uniform Distribution

Desalting with Acetone Solution

Polymeric Precursor

Dried and Calcined at 600 oC/4h

ZnNb2O6 Nano Powders

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280

diameter 14 mm and about 7 mm height at a pressure of 150 MPa. The green compact can be

heat treated at different temperatures and the dimension changes are recorded. The density

can be determined each time and the sintering temperature can be optimised as the

temperature which gives the maximum densification.

The sintered pellets of all the compositions can powdered and the crystalline phase of the

powders are identified by the XRD analysis using Cu-Kα radiation of wavelength (λ)

1.54056Ǻ for 2θ range 10-80o. The recorded patterns are compared with standard ICDD Data

file with the help of Philips X’pert High Score plus software. The ZN nano particles can be

characterised using transmission electron microscopy (HRTEM) FEI Technai G2 30S-TWIN

high resolution electron microscope operated at 300 KV. The crystallite size, lattice

parameter and the selected area diffraction patterns can be recorded using TEM. The

crystallite size (d) of the nano-ZN was determined from the XRD patterns using Debye

Sherrer formula [18,19],

0.9

cosd

λ

β θ= (1)

where λ is the wavelength of the x-ray, β is the FWHM of the maximum intense peak and

θ is the glancing angle.

The microstructure analysis of the sintered polished and thermally etched samples can be

carried out using scanning electron microscope (SEM, JEOL-JSM, 5600LV, Tokyo, Japan).

The bulk densities of the sintered pellets can be measured by the Archimedes method. The

dielectric constant can be measured using the post resonator method of Hakki and Coleman

modified by Courtney. The unloaded quality factor can be measured by a resonant copper

cavity whose interiors are coated silver and the ceramic composites are placed on a low loss

quartz spacer which reduces the effect of losses due to surface resistance of the cavity using

a Vector Network Analyser. The temperature coefficient of resonant frequency (τf) can be

measured by noting the temperature variation of the same using TE01δ mode in the

transmission configuration over a range of temperature 20-80oC. The temperature

coefficient of the resonant frequency can be calculated using the following relation in a fixed

interval of temperature [20-22],

( )

2 1

1 2 1f

f f

f T Tτ

−=

− (2)

where, 1f and 2f are the resonant frequencies at temperatures 1T and 2T respectively and

the average value can be calculated and reported.

3. Observations and analysis

Fig 2(a) is the powder XRD diffraction pattern of ZnNb2O6 synthesized using solid state

ceramic route. All the peaks are compared with the ICDD file card for ZN (Number 76-1827)

and indexed. Fig 2 (b) depicts the XRD pattern of ZnNb2O6 with 5wt% of zinc borosilicate

glass (ZBS). The addition of ZBS glass did not produce any additional phases, as evident

from Fig. 2 (b).

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Fig. 2. XRD pattern of ZnNb2O6 (a) sintered at 1200oC/2h and (b) ZnNb2O6 + 5Wt%

The powder XRD patterns of the calcined ZnNb2O6 nanopowders are depicted in the fig 3.

Fig 3(a) shows the XRD pattern of ZnNb2O6 calcined at 600oC/4h and fig 3 (b) is that of

ZnNb2O6 powder sintered at 950 oC/2h. The figures clearly indicate that the powder

patterns are in well accordance with ICDD data card (76-1827). The average crystallite size

of the nanostructured ZN calcined at 600 oC/4h can be estimeted from the X-ray diffraction

pattern. The fig 4 depicts the maximum intense peak obtained from XRD of ZN ceramics.

Using Gaussian fit (as seen in the fig 4), the FWHM and centre of the peak can be

determined. Employing Debye Sherrer formula (equation 1) the average crystallite size can

be calculated as ~17 nm.

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Fig. 3. XRD pattern of ZnNb2O6 (a) calcined at 600oC/4h and (b) calcined at 950oC/2h.

Fig. 4. Maximum intense peak in XRD pattern of nanostructured ZN ceramics with theoretical Gaussian Fit

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Fig 4 shows the various TEM images of ZN nanocrystallites. Fig 5 (a), (b) and (c) are the

TEM images of the ZN nanocrystallites at two different regions. It can be seen that in both

images the nanocrystallites have same characteristics and mostly of spherical in shape. The

fig 5 (c) establishes the crystallite size at low magnification; they are well separated and have

uniform size distribution. A histogram is shown in fig 5 (d), indicates the crystallite size

obtained from the images both fig 5 (a) and (b). In order to obtain particle size distribution, a

Gaussian function is fitted for the experimental data. The average particle size is obtained

and found that it is lies between 18-20 nm. This is in good agreement with the crystallite size

obtained from XRD using Debye Scherrer formula.

Fig. 5. TEM images of nano ZnNb2O6 (a), (b) 50 nm scale, (c) 100 nm scale and (d) histograph of particle size distribution obtained from image (a) and (b).

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The high resolution TEM (HRTEM) image and the selected area diffraction pattern (SADP)

of ZN nanocrystallites are shown in the fig 6 (a) and (b) respectively. Lattice plane of ZN

nanoparticles are clearly visible in the HRTEM image. Inter planar spacing ‘d’ of several

planes were determined using set of Fourier Transforms of lattice fringe images. TEM image

analysis software (Digital Micrograph - Gatan) was used for the determination of

interplanar spacing. The average d values were determined and the plane corresponding to

each set of fringes are duly indexed, shown in the fig 6 (a). Interplanar spacing obtained

from TEM and XRD are tabulated in table 2. Comparing the d values from the X-ray

diffraction pattern, the planes corresponding to each ring was identified and indexed.

Fig. 6. (a) HRTEM of nanostructured ZN ceramic (b) SAD patterns of ZN ceramics.

In fig 7, variation of bulk density of pellets, synthesised by solid state ceramic route and heat

treated at 975oC with different wt% of ZBS is shown. From the fig 7, ZN ceramics with 5

wt% of ZBS has maximum density. Hence 5 wt% of ZBS is taken as the optimized glass

addition amount for the composite. Fig 8 illustrates the variation of the bulk density of ZN

and ZN-ZBS glass composites with varying sintering temperatures. From Fig. 8 it can be

seen that nanostructured ZN with 5 wt% of ZBS glass has greater density at lower sintering

temperatures (925oC/2h). Comparison of densification of pure (nano and micron sized) ZN

ceramics shows that the densification is faster for nano powder compacts at lower

temperatures, however at temperature above 1100oC for micron sized powder compacts

synthesised by solid state ceramic route has higher absolute density. Since the nano

powders of ZN have higher sinterability, the growth during the sintering process will be

more rapid. Hence more finer intergranular porosities will be formed in the case of nano

powder compacts than micron sized powder compacts. This will results in the reduction in

the values of absolute densities of sintered powder compacts. Similar effects were noticed

for sintered nano sized powder compacts of BZT by Manoj Raama Varma et al. [23] In

nanostructured ZN ceramics the grain boundary area per unit volume will be more than

that of the solid state synthesised ZN ceramics [24]. This deteriorates the densification of

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Fig. 7. Variation of bulk density of ZN ceramics (solid state synthesis) with different wt% of ZBS glass

Fig. 8. Variation of the bulk density of ZN ceramics and ZN-glass composites with different temperature

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nano sized ZN ceramics. In the case of glass addition, low melting glasses such as ZBS enhances the sinterability of the ZN ceramic powders due to the liquid phase sintering [25,26]. The glasses start melting at lower temperatures and the molten glasses flows through the porosity between the grains and fill the porosity and the gap between the grain Boundaries [27]. This enhances the sinterability i.e. the composite gets densified at lower sintering temperatures. Hence the ZN composite with 5 wt% ZBS glass shows higher density than that of the pure materials. The effect of liquid phase sintering is clearly seen in the SEM micrographs ( Fig. 9) as molten glass melted at low temperatures. Fig 9 shows the SEM micrographs of the sintered ZN. Fig 9 (a) and (b) are SEM micrographs of ZN synthesised using solid state ceramic route. Large grains having average grain diameter of ~4.2 µm were observed for pure ZN. Though the sintering has taken place,

Fig. 9. SEM micrograph images of ZN ceramics and ZN-glass composites

Fig. 9 (b) shows a highly densified microstructure with large amount of molten glass phases for glass-ZN composites. It is observed that by the addition of ZBS glass the grain growth decreases. In fig 9 (b) the grains have an average diameter of about 1 to 2 µm only. Fig 9 (c) shows the SEM images of the sintered nano structured ZN (obtained via chemical synthesis) which are sintered at 925 oC for 2h. The SEM micrograph exhibits highly dense grains. Average grain size obtained from the micrograph images ranging from about 1-2 µm. It reveals fine sintered grains are obtained by sintering of nano ZN powder. Fig 9 (d) shows micro structure of the nanostructured ZN ceramic powders with 5wt% of ZBS glass. Comparatively smaller grains are obtained for the glass added nano powder compacts of ZN. The densification is very high for a much lower sintering temperature viz 925oC/2hrs.

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Comparing all images in Fig. 9 the nanostructured ZN with 5wt% of ZBS shows very high packing with a minimum porosity at 925 oC/2h.

The density and the dielectric properties viz dielectric constant rε , Qxf, and τf, of ZN

ceramics are tabulated in table 2. Comparison of these properties of the materials

synthesised by different preparation techniques reveals that the solid state synthesised

shows greater values for dielectric constant and Qxf. This can be correlated with the effect of

grain size. In ceramic materials the increase in the grain size deteriorates the dielectric loss.

Reduction in the number of grains per unit volume, decreases of grain boundaries per unit

volume and it would result in a material with a lower dielectric loss and better polarisability

which improves both rε and Q×f [28,29]. From the SEM micrographs (Fig. 9) it can be

concluded that the nanostructured ZN has more grain boundaries than solid state

synthesised ZN and hence had a low rε and Q×f. However the glass addition to these

compounds, due to liquid phase sintering the nanostructured ZN has greater densification

at lower sintering temperature. The dielectric properties of micron sized ZN cermic with 5

wt% of ZBS are: density =5.48, rε = 21.3, τf = -66 ppm/oC and with Q×f ~38,000, sintered at

975 oC/2h. Dielectric properties of sintered nano sized ZN with 5 wt% of ZBS have density

= 5.21, rε = 22.5, τf = -69.6 ppm/oC and with Q×f ~12,800, sintered at 925 oC/2h [30]. Since

ZnNb2O6+5wt%ZBS can be identified as one of the potential LTCC materials sintering at

925oC, co sintering studies were done with silver. ZnNb2O6+5wt%ZBS was mixed with 20wt%

metallic Ag (99.99%) and sintered at 930oC/2h. SEM pictures with EDAX was recorded after

sintering and found that Ag is not reacting or melting during the sintering. Hence the co

sintering of ZnNb2O6+5wt%ZBS+20wt%Ag was successful as can be seen in fig.10.

hkl Plane d spacing from TEM (Ǻ) d spacing from ICDD file(Ǻ)

111/310 3.6494 3.3602

311 2.9559 2.8455

020 2.8630 2.6789

002 2.5200 2.3098

312 2.0736 1.9918

131/330 1.7703 1.5676

313 1.5260 1.3099

041 1.3770 1.1561

Table 1. d spacing of major reflecting planes determined from TEM analysis

Materials Sintering

temperature (oC)

Density (ρ) (g/cc) rε Qxf

(GHz)

τf

(ppm/o

C) ZnNb2O6 – solid state

synthesis1200 5.32 23.3 12,800 -77.9

ZnNb2O6 + 5wt% ZBS 975 5.48 21.3 38,000 -66.0 ZnNb2O6 – polymer complex synthesis

1200 4.87 19.2 77,900 -66.4

ZnNb2O6 + 5wt% ZBS 925 5.21 22.5 12,800 -69.6

Table 2. Density and microwave dielectric properties of ZN and ZN-glass composites

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Fig. 10. SEM with EDAX micrographs of ZnNb2O6+5wt%ZBS+20wt% metallic Ag-Sintered at 930oC/2h

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4. Conclusions

Zinc Niobates ceramics were prepared in phase pure powder form using solid state ceramic technique and polymer complex method. Particle size of the ZN ceramics is determined using TEM and it is found most of the particles are in the range 18-20 nm, the particle size obtained from XRD pattern using Debye Sherrer formula is 17 nm. Effect of particle size on the sinterability and the microwave dielectric properties were studied. Micro structure shows that a high density ZN ceramics can be obtained by sintering nanopowder of ZN with 5wt% of ZBS glass at 925 oC for 2h. Optimized sintering of nano sized powder at 925 oC/2h give microwave dielectric properties of rε =22.5, Qxf~12,800 and τf= -69.6 ppm/oC.

These composites were successfully co sintered with silver.

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Characterization and Microwave Dielectric Properties of M2+Nb2O6 Ceramics, Robert C. Pullar, Jonathan D. Breeze, Neil McN. Alford, J. Am. Ceram. Soc., 88 [9] 2466–2471 (2005)

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Low sintering ceramic materials based on Bi2O3-ZnO-Nb2O5 compounds, E. A Nenasheva, N.F Kartenko, J. Eur. Ceram. Soc 26, 1929-1932 (2006)

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Synthesis and Microwave dielectric properties of Zn1+xNb2O6+x, A.G Belous, O. V Ovchar, A. V Kramarenko, B. Jancar, J Bezjak, D. Surnov, Inorg. Mater. 43, [3] 227-280, (2007).

Synthesis, characterization and microwave dielectric properties of ATiO3 (A=Co, Mn, Ni) ceramics. P.S. Anjana and M T Sebastian. J. Am. Ceram. Soc. 89 (2006) 2114-2117.

Effects of ZnCl2 addition on the ZnNb2O6 powder synthesis through molten salt method, Guo Liangzhai. Dai Jinhui, Tian Jintao, Zhu Zhibin, He Tian, Qu Xiaofei, Liu Zhongfang, Mater. Chem. and Phys. 105, 148-150 (2007).

H. Jantunen, R. Rautioaho, A. Uusimaki, S. Leppavuori, J. Eur. Ceram. Soc., 20 (2000), 2331-36. M. T. Sebastian, H. Jantunen, Int. Mater. Rev., 53, (2008), 57-90. M Laren, C. B. Ponton, J. Mater. Sci., 33 (1998), 17-22. O. Renoult, J. P.Boilot, F. Chaput, R. Papiernik, L. G. Hubert-Pfalzgraf, M. Lejeune, J. Am.

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Finite size effect on Sinterability and Dielectric Properties of ZnNb2O6 – Glass Composites, Mukkuttiparambil Ayyappan Sanoj, Chalappurath Pattelath Reshmi and Manoj Raama Varma, J. Am. Ceram. Soc. 92(11):2648-2653;Nov 2009

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Advances in Ceramics - Synthesis and Characterization,Processing and Specific ApplicationsEdited by Prof. Costas Sikalidis

ISBN 978-953-307-505-1Hard cover, 520 pagesPublisher InTechPublished online 09, August, 2011Published in print edition August, 2011

InTech EuropeUniversity Campus STeP Ri Slavka Krautzeka 83/A 51000 Rijeka, Croatia Phone: +385 (51) 770 447 Fax: +385 (51) 686 166www.intechopen.com

InTech ChinaUnit 405, Office Block, Hotel Equatorial Shanghai No.65, Yan An Road (West), Shanghai, 200040, China

Phone: +86-21-62489820 Fax: +86-21-62489821

The current book contains twenty-two chapters and is divided into three sections. Section I consists of ninechapters which discuss synthesis through innovative as well as modified conventional techniques of certainadvanced ceramics (e.g. target materials, high strength porous ceramics, optical and thermo-luminescentceramics, ceramic powders and fibers) and their characterization using a combination of well known andadvanced techniques. Section II is also composed of nine chapters, which are dealing with the aqueousprocessing of nitride ceramics, the shape and size optimization of ceramic components through designmethodologies and manufacturing technologies, the sinterability and properties of ZnNb oxide ceramics, thegrinding optimization, the redox behaviour of ceria based and related materials, the alloy reinforcement byceramic particles addition, the sintering study through dihedral surface angle using AFM and the surfacemodification and properties induced by a laser beam in pressings of ceramic powders. Section III includes fourchapters which are dealing with the deposition of ceramic powders for oxide fuel cells preparation, theperovskite type ceramics for solid fuel cells, the ceramics for laser applications and fabrication and thecharacterization and modeling of protonic ceramics.

How to referenceIn order to correctly reference this scholarly work, feel free to copy and paste the following:

Manoj Raama Varma, C. P. Reshmi and P. Neenu Lekshmi (2011). Sinterability and Dielectric Properties ofZnNb2O6 – Glass Ceramic Composites, Advances in Ceramics - Synthesis and Characterization, Processingand Specific Applications, Prof. Costas Sikalidis (Ed.), ISBN: 978-953-307-505-1, InTech, Available from:http://www.intechopen.com/books/advances-in-ceramics-synthesis-and-characterization-processing-and-specific-applications/sinterability-and-dielectric-properties-of-znnb2o6-glass-ceramic-composites

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